Boggle coding challenge

class TreeNode(object):
    def __init__(self, prefix, parent=None):
        self.prefix = prefix
        self.branches = {}
        self.word = False
        self.parent = parent

    def add(self, item):
        if item == self.prefix: # done
            self.word = True
            return

        letter = item[len(self.prefix)]
        if letter not in 'abcdefghijklmnopqrstuvwxyz':
            return # not alphabet, nevermind

        if letter not in self.branches:
            self.branches[letter] = TreeNode(item[0:len(self.prefix) + 1], self)
        self.branches[letter].add(item)

    def dump(self, pad=''):
        print "%s%s %s" % (pad, self.prefix, self.word,)
        for letter in self.branches.iterkeys():
            self.branches[letter].dump(pad + ' ')

class Dictionary(TreeNode):
    def __init__(self):
        super(Dictionary, self).__init__('')
        print "loading dictionary ..."
        for word in file('/usr/share/dict/american-english'):
            word = word[0:-1].lower()
            if len(word) > 1:
                self.add(word)
        print "loaded."

def solvePuzzle(dictionary, puzzle):
    results = []
    visited = {}

    # recurse into puzzle, follow allong in tree-structure dictionary
    def searchPuzzle(wordsofar, x, y, treeloc):
        # off puzzle - return
        if x >= len(puzzle[0]) or x < 0:
            return
        if y >= len(puzzle) or y < 0:
            return

        # already used this letter - return
        if "%s-%s" % (x, y) in visited:
            return

        wordsofar = wordsofar + puzzle[y][x]
        treeloc = treeloc.branches.get(wordsofar[-1])

        # no words in dictionary from this stem, give up
        if not treeloc:
            return

        # new word found
        if treeloc.word and wordsofar not in results:
            results.append(wordsofar)

        pos = "%s-%s" % (x, y)

        visited[pos] = True
        searchPuzzle(wordsofar, x+1, y, treeloc)
        searchPuzzle(wordsofar, x-1, y, treeloc)
        searchPuzzle(wordsofar, x, y+1, treeloc)
        searchPuzzle(wordsofar, x, y-1, treeloc)
        del visited[pos]

    for x in range(0, len(puzzle[0])):
        for y in range(0, len(puzzle)):
            searchPuzzle('', x, y, dictionary)

    return results

if __name__ == '__main__':
    print solvePuzzle(Dictionary(), ['xxdx', 'xxrt', 'xcax', 'xxxx'])